This invention relates generally to airflow measuring assemblies and particularly to airflow measuring assemblies and methods to measure airflow in air handling equipment. The assemblies of the invention are constructed and arranged for placement in an air handling system and to condition or direct the airflow of the system into a controlled fluid stream for analysis. The measuring assemblies and methods provide accurate and reproducible airflow measurements for the air handling equipment being monitored or tested. More particularly, the assemblies of this invention are constructed and arranged for use in forced air HVAC distribution systems. The airflow measuring assembly is constructed to be placed in a predetermined location of a forced air distribution system, for example, in a slot proximate the air handler unit.
It is important for air handling equipment, such as furnaces, heat pumps and air conditioners to have proper airflow to insure efficient operation of the HVAC systems. Specified airflow or air volume rates are required to be within specified airflow ranges for particular air handling systems. The flow of air is typically measured to insure that a forced air distribution system, for example, was properly designed and installed, and is operating according to the specification. For example, it is important to avoid low airflow in heat pumps, furnaces and air conditioners because reduced airflow results in performance inefficiency and can damage the compressor. Determining an unusually low or high airflow may indicate that a leak in the system or insufficient ventilation exists.
A forced air distribution system typically includes an air handler unit, ductwork, registers, dampers, filters, etc. Although the airflow measuring assembly may be utilized in connection with any air handler unit, which contains the blower, the assembly may also be utilized at any location within the air distribution system. Preferably, the assembly is used in the filter position of the air distribution system. Depending upon the similarity of flow resistance between the assembly and the filter typically installed in the air distribution system, a correction factor is calculated and used in the method of the invention.
Airflow is preferably measured, however, proximate the air handler unit of the air handling assembly, where proper airflow is important for efficient operation of the equipment. Although the airflow measuring assembly may be positioned, permanently or removably, at a number of locations in an air handling assembly, it has been found that the air filter slot typically provided in air handling equipment provides an efficient, easy and convenient place for such airflow measurement. However, it is within the purview of the invention to utilize other positions or specified slots in an air handling assembly to make such airflow measurements. Thus, the airflow measuring assembly of the invention may be pre-installed in a furnace proximate the blower, may be positioned for use in a specified slot across the airstream, may be positioned in the filter slot of air handling equipment and in the filter slot of a forced air distribution system, whether proximate the blower unit or remote therefrom.
In residential furnaces, for example, a specified volume of air is heated and distributed throughout a house and is returned to the furnace by the air return duct system. It is desirable to measure the flow of air through the furnace, in order to insure airflow within the specified operating range of a furnace. The airflow measuring assemblies of this invention measure airflow in a furnace, such as at the filter position, and are constructed to be placed into the furnace filter slot or a similar position in the furnace. This placement has been found to provide a convenient, accessible location for the accurate air volume flow rate measurements. The airflow measuring assemblies are constructed and arranged to condition and direct the airflow therethrough and to measure the airflow via the use of an anemometer, for example, or by measuring a pressure signal generated by the assembly. In the latter assembly, a differential pressure signal generated from single or multiple designed pressure signal locations within the controlled fluid stream can be obtained and converted into volumetric airflow utilizing mathematical relationships. The assemblies of the invention may also be constructed and arranged to simulate the resistance of a typical filter for accuracy and reproducibility of the airflow measurements. Furthermore, a correction factor based on air handler system pressures measured with a typical filter, and again, with the assembly can be derived for improved accuracy. The correction factor accounts for the difference in airflow resistance between the test assembly and filter and is used to correct the measured airflow of the system by the test assembly.
Although the utilization of the airflow measuring assembly of the invention at or near the air handler may be preferred, the assembly may also be used at other locations of an air handler assembly. For example, filter holders may be built into ductwork or positioned at the return grille of the system. If the assembly is installed remotely from the furnace or air handler, duct leakage between the assembly and the air handler may exist and needs to be taken into consideration. Further, if the pressure drop across the filter is significantly different from that of the airflow measuring assembly, this difference is taken into consideration by calculating a correction factor.
Various airflow measuring assemblies and methods to estimate volumetric airflow have been proposed and used in the past, however, these prior art assemblies and methods have drawbacks and difficulties. Prior art methods and devices have often been found to be burdensome, difficult to use and yield inaccurate and unrepeatable results for purposes of measuring airflow in air handling equipment. For example, prior art devices typically require more care in proper use to obtain accurate results than installers or others have time to provide. Prior art assemblies and techniques include the utilization of heating elements in an air stream and a calibrated fan assembly used to deliver air to the return side of the air handler. However, accuracy in result and the time consumption required, to utilize these prior art techniques are undesirable for general testing usage. Other prior art assemblies use pitot tubes or like devices to measure air velocity pressures at various locations in a system and convert the measurement to volumetric airflow. Other prior art assemblies measure airflow at air outlets by trapping air in large capture hood devices. Capture hood devices are often difficult to seal over registers and return grilles due to obstructions and their respective locations, i.e., due to furniture, under cabinet locations, etc. And, because capture hood devices measure airflow at the registers and return grilles, any duct leakage is not accounted for when airflow at the air handler is desired.
Still other prior art devices attempt to have as little effect as possible on air flow by minimizing the pressure loss through the measuring device. The latter devices tend to be very sensitive to upstream conditions and require long lengths of straight duct to be accurate. Such lengths of straight duct often do not exist in actual installed systems. On the other hand, the assembly and method of this invention have been found through experimentation to be insensitive to upstream conditions. In summary, known prior art assemblies and methods are difficult and cumbersome to use and do not necessarily provide accurate airflow measurement at the air handler, where such measurement is most useful and yield more accurate and reproducible results.
Prior art assemblies, as far as is known, have not been developed to measure airflow at the air filter location of an air handler, for example. The airflow measuring assembly of the present invention overcomes the difficulties in use and inefficiencies of the prior art. The measuring assemblies of the present invention provide fast, easy and accurate means to measure the airflow in a forced air HVAC system or the like by placement of the assembly into the filter section or a similar position within the system.
Another benefit of the measuring assembly of the present invention is that it permits a more accurate derivation of duct efficiency. Duct efficiency equations are generally dependent upon airflow through the air handler. The accurate measurements provided by the present invention thereby yield more accurate calculations of duct efficiency.
It is a further object of this invention to provide airflow measuring assemblies which are adjustable in size to accommodate various air handler filter slot dimensions.
The present invention relates to an airflow measuring assembly and method which conditions and directs a fluid stream for analysis. The invention further relates to an airflow measuring assembly having airflow sensors for providing a differential pressure signal to determine volumetric airflow in air handling equipment. The assemblies are constructed and arranged to be positioned into a filter slot, for example, or other position of an air handler system. The assembly in a filter slot preferably approximates the filter resistance in conditioning the airflow to provide easy and accurate airflow measurements. Airflow correction factors are calculated in the method of the invention. Correction factors either based on filter parameters or measured at the time of testing may be utilized. For example, charts or tables with empirical data may be developed with correction factors for various models and sizes of filters. Alternatively, to correct for the change in airflow at the time of testing, a representative static pressure in the air handling system is measured both when the filter is in place and again when the measuring assembly is in place. For example, for two static pressure readings P1 and P2, respectively, the corrected flow is the flow through the test assembly multiplied by the square root of P1/P2.
The airflow measuring assemblies of the present invention are comprised of a plate structure having at least one aperture therethrough to precondition and direct an airflow stream for measurement. The plate structure is preferably adjustable in area to accommodate various air handler slots, such as filter slots, or duct sizes. The plate structure is constructed so that inserts or extensions may be added to the assemblies to insure a proper fit and minimal leakage. The plate structure may have a plurality of apertures with airflow sensors and may utilize various sensor and aperture patterns to precondition and direct airflow and to approximate filter resistance to thereby provide accurate and measurable airflow streams.
The airflow measuring assembly of the invention is constructed and arranged to provide a pressure differential measurement comprised of an upstream pressure signal and a downstream pressure signal which are generated from designed pressure signal locations in the controlled airflow stream. The upstream pressure signal may be equated to the total pressure in the air stream before going through the conditioning aperture of the assembly. The downstream pressure signal may be equated to the static pressure after conditioning, which depends on the volumetric flow rate, plate aperture geometry, fluid density and viscous losses. The differential between the total pressure and the static pressure is defined as the dynamic pressure. In this case, the readily measurable dynamic pressure correlates to the airflow of the fluid distribution system.
Airflow measuring means or air pressure sensors are positioned within each plate aperture to measure airflow. For example, an anemometer may be used to measure airflow and alternatively, total and static pressure sensors, which may be interconnected to provide an average value of the respective pressure signals, may also be used to provide the dynamic air pressure and thus provides the airflow measurement. In the latter method, apertures in the air pressure sensors to measure total and static pressure are disposed behind the plane of the plate structure. The assemblies of the latter invention are provided with connectors for attachment to a differential pressure manometer. The manometer provides the difference between the total and static pressure signals and from which airflow may be calculated.
To provide an accurate and repeatable volumetric airflow rate for air handling equipment is the primary object of the present invention. The flow plate provides a controlled fluid flow stream which may be measured by various sensors which can be converted to volumetric airflow. For example, using a mathematical relationship, the differential pressure signals may be converted to volumetric airflow. In summary, the volumetric airflow rate provided by the present invention aids in verifying that the air handling equipment was properly designed and installed and is operating within the airflow range specified for efficient use. The volumetric airflow rate can be used as inputs to calculate the duct and/or furnace efficiency.
These and other benefits of this invention will become clear from the following description by reference to the drawings.